Given that American Express and Discover aren't accepted in as many places as this duopoly and they've both been around for 30+ years, that tells you everything you need to know.
I can’t speak to Discover but Amex is so annoying to deal with. You have to set them up separately (from MC and Visa), they require you to setup a merchant account with them to accept Amex (payment processors like Stripe do this for you but if you go any lower-level you have to do some setup).
Then the Amex reps call you can try to spout the benefits of accepting Amex, how they can advertise your business to their card holders, yada, yada. It all feels like bullshit. I have an Amex card, I’ve never visited or not visited a business due to them taking or not taking Amex. I’ve never cared to look at the Amex site to find places that accept it.
They want to make it feel exclusive but it’s just not.
